1. Field of the Invention
This invention relates to a method of forming a chemical phosphate coating, such as of zinc phosphate, on the surface of steel.
2. Description of the Related Art
A chemical phosphate coating is used, for example, as an undercoat for rustproofing a steel plate or improving the adhesion of a paint thereto, or for improving the lubricating property of a frictionally slidable structural steel member. A chemical phosphate coating has hitherto been formed by employing a treatment bath having a temperature of at least 40.degree. C. and supplying it with those quantities of a principal component containing phosphoric acid ions and zinc or other metal ions and an auxiliary component containing nitrite ions which are determined by the chemical volumetric analysis of the total acid, free acid and oxidizing agent in the bath with the aid of the worker's experience. It has, however, been difficult to control the bath properly even with the aid of the worker's experience, as the chemical volumetric analysis requires a lot of time and gives rise to a certain change, which is apparently due to an abnormal reaction, in the bath. It has been difficult to always form a coating of equally good quality which is, for example, useful for rustproofing a steel plate. These problems have been due to the fact that the reaction by which a phosphate coating is formed is not clear in detail and cannot, therefore, be controlled efficiently.